1. Field of the Invention
The present invention is directed to a method of determining the physical location of undesired resonance points or zones induced by acoustic feedback in a spring reverberation unit and to an improved spring reverberation unit in which this acoustical feedback is substantially eliminated.
2. Prior Art
Spring reverberation units have been used in electronic musical instruments and sound systems for many years. In its simplest form, a spring reverberation unit comprises an electro-mechanical transducer connected to one end of a spring and a mechanical-electrical transducer connected to the other end of the spring. This transducer-spring assembly is mounted inside a U-shaped housing or channel. This U-shaped inner channel is mounted by suspension springs to an outer U-shaped housing. A portion of the electrical signal representative of the audio signal is applied to the electro-mechanical transducer which translates the electrical signal to a mechanical movement of the spring. The mechanical-electrical transducer at the opposite end of the spring converts the mechanical movement of the spring back into an electrical signal. This converted electrical signal, the reverberated signal, is delayed in time by an amount proportionate to the length of the spring and other secondary factors.
In a typical sound system the reverberated signal is combined with the unreverberated electrical signal representative of the audio signal and the combined signal is applied to an output device such as a loudspeaker. The resulting audio signal simulates the type of audio signal that would be produced in a concert hall in which the sound reaching the listener arrives at different times due to reflections from walls and other solid objects.
Spring reverberator units are responsive not only to the proper electrical signal input but also to other ambient conditions which cause the spring to vibrate. The spring of the reverberation unit is typically mounted within a housing or inner channel which is in turn mounted by suspension springs to an outer housing or channel to isolate the spring from spurious vibration caused by external physical movement of the unit. The outer housing or channel is typically secured by screws or other means to some portion of the sound system, but because of the compact nature of the sound systems the reverberation unit is usually in close physical proximity to the sound transducer. Therefore, the principal cause of spurious vibrations in the spring is acoustical feedback from the sound output device, loudspeaker, which causes both the inner and outer housings to resonate, thereby imparting a resonant movement to the spring itself. The spring itself is typically immune to such undesired resonance since the pressure waves created by the sound transducer do not impact the comparatively small cross-sectional area of the spring with sufficient force to cause vibration. The greatest portion of the pressure waves impact upon the surface of the outer channel causing it to resonate. Since the outer channel is in close proximity to the inner channel and is air coupled thereto the movement of the outer channel impacts movement to the inner channel. In addition, the inner channel is also impacted, to a much lesser degree than the outer channel, by the pressure waves from the loud speaker. These pressure waves together with the air coupled movement of the outer channel forces the inner channel to resonate. This complicated interaction between the pressure waves from the loudspeaker impacting the inner and outer housing and the air coupling between the inner and the outer housing causes the spring to move thereby introducing spurious signals into the system.
When the induced resonance in the inner and outer housings is sufficiently large, it causes the spring to vibrate at such a rate that the oscillation of the spring becomes self-sustaining. While any movement of the spring which is not the direct result of the input signal to the electro-mechanical transducer causes distortion of the reverberated signal, the self-sustained oscillation of the spring is particularly detrimental and causes a significant spurious output signal from the spring reverberation unit which manifests itself as a screeching or howling in the audio signal. The undesirable self-sustaining oscillation of the spring can be induced by the pressure waves from the sound transducer impacting upon the surface of the outer housing of the reverberation unit even though no electrical signal input is received by the electro-mechanical transducer of the spring reverberation unit. The screeching or howling in the audio output is extremely disquieting to the listener.
Spring reverberation units of the type discussed herein are best described in U.S. Pat. No. 3,106,610 which issued to Alan Young on Oct. 8, 1963. The Young patent is incorporated herein by reference to describe in detail what is frequently referred to as the standard spring reverberation unit.
At least one attempt has been made in the prior art to eliminate the acoustic feedback from the sound transducer which induces reasonance in the inner and outer channels of the spring reverberation unit with the resulting spurious oscillations of the spring and the howling in the audio output. This prior art solution was to encase the entire spring reverberation unit within a sack made from vinyl or similar material in an effort to block or reduce the acoustical pressure waves before they impacted upon the inner and outer housing of the spring reverberation unit. This attempted solution was not only cumbersome in manufacture and costly, but also failed to effectively eliminate the acoustical feedback from the output transducer.